1. Field of the Invention
The present invention relates to a method and apparatus for accurately defining a healthy cross-section area function for quantitative three-dimensional (3D) analysis using in particular multiple angiographic images.
2. State of the Art
Angiography, in particular coronary angiography, refers to the process of investigating coronary arteries to determine the severity of any narrowing that may exist, such as by finding stenotic arteries. Quantitative Coronary Analysis (QCA) of single arteries has become a standard for guiding interventions and executing trail studies in coronary revascularization. However, angiographic analysis of lesions in or near a bifurcation presents a considerable problem since QCA for single arteries cannot handle more complex geometries. The definition of bifurcation herein is a splitting of a main tubular artery into two or more further tubular arteries. For example, the left coronary artery bifurcates into the left anterior descending artery and the left circumflex artery.
QCA of a bifurcation involves automatic segmentation of the bifurcation. This can be followed by reconstructing a healthy state that includes the bifurcation area itself. The reference artery diameter, which means the artery's healthy diameter as computed by the QCA, is typically based on averages of the artery “normal” parts before and after the bifurcation, respectively. So the greatest challenge for bifurcation lesion analysis is extracting the true reference artery size of both the proximal artery and its side branches. Conventional QCA reconstructs the healthy artery by assuming minimal artery tapering and cannot handle large steps in diameter that are caused by the bifurcation itself. Furthermore, conventional QCA focuses on 2D quantitative analysis which is sensitive for out-of-plane calibration errors and foreshortening. Foreshortening is a well known phenomenon in quantitative analysis of x-ray images, caused by the fact that x-ray may be seen as a shadow image.
Most conventional QCA methods allow inputting a user-defined reference, which could eliminate the wrong reference definition. However, this reference diameter, and area in case of 3D, would still only be valid on the one side of the bifurcation where the user defined the reference. This option is furthermore little reproducible such as through operator inaccuracy and subjectivity.
Currently no detailed publication has solved the above limitations. O. Goktekin et al, “A new quantitative analysis system for the evaluation of coronary bifurcation lesions: Comparison with current conventional methods”, Catheterization and Cardiovascular Interventions 69:172-180 (2007), evaluates a bifurcation package, in which the bifurcation is divided into three parts on each of which conventional QCA is applied. Goktekin describes a method for solving the reference problem by eliminating the central bifurcation region from the reference calculations. Therefore, the central bifurcation is still left out of consideration, both for calculating the artery diameter, and also for definition of the artery reference diameter. Furthermore, the method fully focuses on 2D quantitative bifurcation analysis.
U.S. patent application Ser. No. 11/845,154 describes a method to solve the reference problem, but only for 2D quantitative bifurcation analysis. In the present invention the artery reference, which is in 3D expressed in cross-sectional area function is presented. By using 3D information, errors in artery dimension caused by foreshortening and out-of-plane calibration are eliminated. Also the size of the artery along the length of the selected artery segment is expressed in cross-sectional area instead of diameters.
Furthermore in the disclosure hereinafter, several medical terms, such as stenosis, plaque, obstruction and lesion, are used to indicate various medical aspects of a diseased vessel of which ultimately the size must be calculated, without such medical aspects relating to the technical steps and apparatus of the invention.